SERBIAN JOURNAL OF ELECTRICAL ENGINEERING Vol. 7, No. 2, November 2, 67-84 UDK: 62.33.333:68.586.7 Real Time Implementation of Adaptive Sliding Mode Observer Based Speed Sensorless Vector Control of Induction Motor Karim Negadi, Abdellah Mansouri 2, Belkheir Khtemi Abstract: Sensorless induction motor drives are widely used in industry for their reliability and flexibility. However, rotor flux and speed sensors are required for vector control of induction motor. These sensors are sources of trouble, mainly in hostile environments, and their application reduces the drive robustness. The cost of the sensors is not also negligible. All the reasons lead to development of different sensorless methods for rotor flux and mechanical speed estimation in electrical drives. The paper deals with the speed estimators for applications in sensorless induction motor drive with vector control, which are based on application of model adaptive, based sliding mode observer methods. This paper presents the development and DSP implementation of the speed estimators for applications in sensorless drives with induction motor. Keywords: Sensorless control, Induction motor drives, Sliding mode observer, MRAS, Modeling, Field oriented control, Dspace. Introduction Induction motor drives have been thoroughly studied in the past few decades and many vector control strategies have been proposed, ranging from low cost to high performance applications. In order to increase the reliability and reduce the cost of the drive, a great effort has been made to eliminate the shaft speed sensor in most high performance induction motor drive applications []. Speed estimation is an issue of particular interest with induction motor drives where the mechanical speed of the rotor is generally different from the speed of the revolving magnetic field. The advantages of speed sensorless induction motor drives are reduced hardware complexity and lower cost, reduced size of the drive motor, better immunity, elimination of the sensor cable, increased reliability and less maintenance requirements. The induction motor is however relatively difficult to control compared to other types of electrical motors. For high performance control, field oriented control is the Physics Engineering Laboratory, Ibn khaldoun University of Tiaret, Algeria, E-mail: negadi_k@yahoo.fr 2 Laboratory of Automatics and Systems Analysis, Department of Electrical Engineering, E.N.S.E.T. Oran, Algeria, E-mail: ammansouri@yahoo.fr 67
K. Negadi, A. Mansouri, B. Khatemi most widely used control strategy [2]. This strategy requires information of the flux in motor; however the voltage and current model observers are normally used to obtain this information. Generally, using the induction motor state equations, the flux and speed can be calculated from the stator voltage and current values. This paper presents model adaptive based sliding mode observer for the proposed control. 2 Motor Model and Vector Control Strategy The induction motor mathematical model in d-q coordinates established in a rotor flux oriented reference frame can be written as [3]: d Ψ sd vsd = Rsisd + ωeψsq () dt d Ψ sq vsq = Rsisq + +ωeψsd (2) dt d Ψ rd = Ri r rd+ ωslψrq (3) dt d Ψrq = Ri r rq+ +ωslψrd (4) dt where the stator and rotor flux linkages are given by: Ψ sd = Li s sd + Lmi rd (5) Ψ sq = Li s sq + Lmi rq (6) Ψ rd = Li r rd + Lmi sd (7) Ψ rq = Li r rq + Lmi sq (8) The state space representation of the induction motor with the stator currents and the rotor flux linkages components as state variables can be written as [4]: d isd dt d i sd σls i sq dt isq vsd = A +, (9) d Ψ rd σls Ψ vsq rd dt Ψ rq d Ψ rq dt 68
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... where R s σ Lm Lm + ωe. ωr σls σtr σlslt r r σlslr R σ s Lm Lm ω +. ω e r σl σ σ σ = s Tr LL s r LLT A s r r, Lm ( ω ω ) e r T r Tr Lm ω ( e ωr) Tr Tr T r is the rotor time constant and is given by Tr = Lr R r and σ is the leakage 2 Lm coefficient given by σ= LL. r s The electromagnetic torque and the rotor speed are given by: 3 Lm Tem = p ( Ψrdisq Ψrqisd ), () 2 Lr where: R s and Rr are the stator and rotor winding resistances; L s, Lm and Lr are the stator, mutual and rotor inductances; p is the number of pole pairs; ω e, ω r and ω sl are the synchronous, rotor and slip speed in electrical rad/s; v sd, v sq, i sd, i sq, Ψ rd and Ψ rq are stator voltage, stator current and rotor flux d-q components in the rotor flux oriented reference frame; T em and T l are the electromagnetic torque and the load torque respectively; J and f are the motor inertia and viscous friction coefficient respectively. Under the rotor flux orientation conditions the rotor flux is aligned on the d-axis of the d-q rotor flux oriented frame and the rotor flux equations can be written as: Ψ rq = () Ψ = rd The electromagnetic torque equation can be written as: 69 Li m sd (2)
K. Negadi, A. Mansouri, B. Khatemi 3 Lm Tem = p Ψ risq = Ktisq, (3) 2 Lr where K t is the torque constant given by: 3 L K = m t p Ψ r. (4) 2 L 3 Model Reference Adaptive Systems (MRAS) The basic concept of MRAS is the presence of a reference model which determines the desired states and an adaptive (adjustable) model which generates the estimated values of the states. The error between these states is fed to an adaptation mechanism to generate an estimated value of the rotor speed which is used to adjust the adaptive model. This process continues till the error between two outputs tends to zero [4, 5]. Basic equations of rotor flux based- MRAS can be written as: L r Ψ = ( )d r vs Rsis t Li s s, (5) Lm ˆ Ψ = ( Ψˆ ωˆ Ψˆ r Li m s r Tr r)dt. (6) Tr The reference model (5) is based on stator equations and is therefore independent of the motor speed, while the adaptive model (6) is speeddependant since it is derived from the rotor equation in the stationary reference frame. To obtain a stable nonlinear feedback system, a speed tuning signal ( ε ω ) and a PI controller [6] are used in the adaptation mechanism to generate the estimated speed. The speed tuning signal and the estimated speed expressions can be written as: ε ˆ ˆ ω =ΨrqΨrd ΨrdΨ, (7) rq Ki ω ˆ = K p + εω, (8) s where K p and K i are the proportional and integral constants, respectively. Generally the MRAS observer gives satisfactory speed estimation in the high and medium speed regions. When working at low speed the observer performance deteriorates due to integrator drift and initial condition problems and sensitivity to current measurement noise. Therefore a sliding mode observer is proposed to replace the conventional adaptive model (6) (Fig. ) to improve 7 r
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... the MRAS scheme performance at load condition, reversal speed and low speed region [7]. v i Reference Model x + Sliding Mode Observer ωˆ 7 - xˆ ε ω Adaptation Mechanism with PI Controller Fig. MRAS with sliding mode observer. 4 The Propose Sliding Mode Observer Considering only the four first equations of the induction motor model and noting xˆ, xˆ 2, xˆ 3, x ˆ 4 the observation of x, x 2, x 3, x 4 respectively. The observer model is a copy of the original system, where added correctors are gains with switching terms [8]. Let, Λ, Λ 2, Λ 3 and Λ 4, are the observer gains with Λ = Λ Λ j j j 2 for j {, 2,3, 4 } Sign ( S ) is =, (9) Sign ( S2 ) with: S x xˆ S = =Γ, ˆ S2 x2 x2 K pωk Tr Γ=, β K pωk Tr 2 K β= + p ω K Tr 2 2 2 The choice of Γ is made to get a simple observer gain synthesis. Setting = ˆ j, 2,3, 4, the estimation error dynamics is given by e x x for { } j j j K e = e + pωke Λ i 3 4 s Tr K e = e pωke Λ i 2 4 3 2 s Tr. (2) (2)
K. Negadi, A. Mansouri, B. Khatemi e = e pωe Λ i 3 3 4 3 s Tr e = e + pωe Λ i 4 4 3 4 s Tr 72 (22) (23) One has the following result. Assume that the state variables x3 and x 4 are bounded, and consider system (2) at (24) with the following observer gain matrices [9]: Λ Λ2 δ =Γ 2 22, Λ Λ δ2 where q Tr Pω Λ3 Λ32 δ δ2 = Λ4 Λ42 q2 pω Tr δ δ2 δ =ρ + xˆ + a e + b e 3 3 max max 2 δ =ρ + xˆ + b e + a e 2 4 4 max max 2 2 2 2 2 amax = 2Tr p K ηη 2, bmax = ptr η 2 + 2 p η, K and η, η 2 are two known positive parameters. Since x 3 ρ3, x 4 ρ4 and q, q 2 >. then [ ] T e e2 e3 e 4 converge to zero. It should be noted that our observer is robust in face of modeling uncertainty and error in the measurements []. The implementation of the nonlinear sliding mode observer is easy using the Embedded MATLAB function block is based on an M-file written in the MATLAB language because it can be supported by RTI (Real Time Interface). 4 The convergence of the algorithm is assured with simple time T e =.5 s, use Euler method.
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... 5 Description of Laboratory Setup The adaptive sliding mode speed observer has been tested experimentally on a suitable test setup (Fig. 2). The test setup consists of the following: Three-phase induction motor with rated values shown in appendix; Synchronous machine for loading the induction motor; Electronic power converter: three-phase diode rectifier and VSI composed of three IGBT modules without any control system; Electronic card with voltage sensors (model LEM LV 25-P) and current sensors (model LEM LA 55-P) for monitoring the instantaneous values of the stator phase voltages and currents; Voltage sensor (model LEM CV3-) for monitoring the instantaneous value of the dc-link voltage; Incremental encoder (model RS 256-499, 25 pulses per round), only for comparison measurements; dspace card (model DS4) with a PowerPC 64e at 4 MHz and a fixed-point digital signal processor (DSP) TMS32F24. Fig. 2 Experimental setup. During the real-time operation of the control algorithm, the supervision and capturing of the signals can be done by the Control Desk software provided with the DSP board. 73
K. Negadi, A. Mansouri, B. Khatemi 6 Experimental Results and Discussion The application of proposed reconstruction technique and estimation of feedback signals is illustrated by a computer simulation. Flux reference is set to its rated value and the dc link voltage is 5 V. Some experimental results were provided to demonstrate the effectiveness of the proposed observer technique. Scenario : First experiment, the target speed is changed from rpm to 49 rpm at.5 s at no load applied. Fig. 3 shows the experimental result of a speed at free acceleration using the estimation of speed. Additionally, the real speed is measured and compared. It can be seen that there is a very good accordance between real and estimated speed without any steady state error. 2 8 6 4 2 8 6 47 46 45 44 43 4 2 42 4 4 2 2. 2.2 2.3 2.4 2.5 2.6-2 5 5 Fig. 3 The reference, actual and estimated speed at no load condition. Fig. 4 depicts the trajectories of the electromagnetic torque (a), estimated and measured stator currents evolution (b). It should be noted that the amplitude of the torque ripple is slightly higher. Scenario 2: Fig. 5 shows the speed sensorless control performance where the load was applied and omitted. The estimated speed coincides exactly with the real speed even the load torque application instant. From these results, it is shown that the proposed speed-sensorless control algorithm has good performances. 74
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed....2 Electromagnetic torque (Nm).8.6.4.2 -.2 5 5 a) 7 6 isqest isqmes 5 Current isq (A) 4 3 2-5 5 b) Fig. 4 a) Response electromagnetic torque; b) Estimated stator currents evolution. 75
K. Negadi, A. Mansouri, B. Khatemi 8 6 4 2 7 8 6 4 6 5 4 3 wref 2 2 wref 6 7 8 9 2 3 4-2 5 5 Fig. 5 The reference, actual and estimated speed at speed applied a load at 5-7-3-5 s. The real and estimated stator currents and the estimated electromagnetic torque, when the motor is running at high speed with load applied, are given by Figs. 6a and 6b. these figures show that the real and the estimated stator currents, the estimates electromagnetic torque are in close agreement. Scenario 3: The reference speed is set to 4 rpm at t = 2s. Then the set point is changed to -4 rpm at t = s without any load (Fig. 7). It is clear that the speed response exhibits good performances at both dynamics regimes. The result clearly shows that the estimated speed follows the actual speed and the error is not significant. The results shown in Figs. 8a and 8b illustrate, the electromagnetic torque, actual and estimated stator currents trajectory. These results made the drive remain stable and this condition can be maintained indefinitely. Scenario 4: The controller was tested under with the speed dependent load produced by the synchronous machine. The reversal speed response of the motor is shown in Fig. 6 at high speeds without load and in Fig. 7 under different levels of load torque. From inspection of Figs. 8a and 8b, it is possible to verify the excellent behavior of the proposed algorithm. In fact, the error on the estimation both of 76
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... the stator currents and of the electromagnetic torque are always very small (<2%, by referring to the actual values). Fig. 7 shows too the error between real and estimated speeds. Figs. 4 and 5 indicate that the estimated value also tracks its true value very closely in both the forward and reverse directions..2 Electromagnetic torque (Nm).8.6.4.2 -.2 5 5 a) 7 6 isqest isqmes 5 Current isq (A) 4 3 2-5 5 b) Fig. 6 a) Response electromagnetic torque; b) Estimated stator currents evolution. 77
K. Negadi, A. Mansouri, B. Khatemi 2 5 wref 5-5 - 6 5 4 3 2 - wref -5-2 -3 9.9..2.3.4.5.6.7.8-2 2 4 6 8 2 4 6 8 2 Fig. 7 The reference, actual and estimated speed at speed reversal with no load condition. Scenario 5: To test the performance of the control drive at low speed without load. We applied a changing of the speed reference from rpm to - rpm at t = s. Speed control performance is shown in Fig.. We can see that the speed follow perfectly the speed reference. However, it is important to note that the control system demonstrates a good performance even under those variations. We note that the performance degrades as approaching the low speed region and fails to provide large oscillations. Figs. 2a and 2b show torque and stator current estimation in the low speed operation. Excellent tracking performance was obtained no study state error and no overshoot and control performance of the drive is acceptable for load disturbance. The gotten results show the effectiveness of the proposed control scheme. 78
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed....5 Electromagnetic torque (Nm).5 -.5 - -.5 2 4 6 8 2 4 6 8 2 a) 8 6 isqest isqmes 4 Current isq (A) 2-2 -4-6 -8 2 4 6 8 2 4 6 8 2 b) Fig. 8 a) Response electromagnetic torque, b) Estimated stator currents evolution. 79
K. Negadi, A. Mansouri, B. Khatemi 2 5 wref 5-5 - 6 4 2 8 6 4 wref 2-5 4 5 6 7 8 9-2 2 4 6 8 2 4 6 8 2 Fig. 9 The reference, actual and estimated speed at reversal speed with a load applied at 5-7-3-5 s..5 Electromagnetic torque (Nm).5 -.5 - -.5 2 4 6 8 2 4 6 8 2 Fig. a Response electromagnetic torque. 8
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... 8 6 isqest isqmes 4 Current isq (A) 2-2 -4-6 -8 2 4 6 8 2 4 6 8 2 Fig. b Estimated stator currents evolution. 5 wref 5-5 6 4 2 98-96 94 wref 92 2.8 3 3.2 3.4 3.6 3.8 4 4.2-5 2 4 6 8 2 4 6 8 2 Fig. The reference, actual and estimated speed at speed applied at low speed with reversal speed. 8
K. Negadi, A. Mansouri, B. Khatemi.5 Electromagnetic torque (Nm).5 -.5 - -.5 2 4 6 8 2 4 6 8 2 a) isqest isq mes.8.6.4 Current isq (A).2 -.2 -.4 -.6 -.8-2 4 6 8 2 4 6 8 2 b) Fig. 2 a) Response electromagnetic torque; b) Estimated stator currents evolution. 82
Real Time Implementation of Adaptive Sliding Mode Observer Based Speed... 7 Conclusion Both observers presented in the paper can estimate the speed in an electrical drive system. The values of the speed estimated on-line can be used by the control of the electrical drive systems in speed range. For both observers the drive system must be endowed an acquisition system for the electrical variables, which helps to calculate the electromagnetic torque of the electrical motor, who is based on the generally equation of motion the same in every drive system for any kind of electrical motor. The estimation performances of both observers are comparable. For the observers presented in this paper the drive s parameters were considered constant. The main features are the following: The instantaneous speed is estimated by Adaptive sliding mode observer. To obtain a high-dynamic current sensorless control, a current to voltage feed forward decoupling and a dynamic correction are applied. Moreover, an accurate dynamic limitation of the real electromagnetic torque is obtained. Extensive experimental results using an induction motor drive prove highdynamic performances and robustness of the proposed control structure in dspace environment. 8 Appendix R s =.8Ω, R r =.385Ω, L s = L r =.5568 H, L m =.6585 H, J =.2 kg m 2, f = 3.65e 4 Nm/rad/s, p=. 9 References [] R. Magurean, C. Ilas, V. Bostan, M. Cuibus, V. Radut: Luenberger, Kalman, Neural Observers and Fuzzy Controllers for Speed Induction Motor, Buletinul Institutului Politehnic Iasi Tomul XLVI (L), FASC 5, 2. [2] A. Bentaallah, A. Meroufel, A. Bendaoud, A. Massoum, M.K. Fellah: Exact Linearization of an Induction Machine with Rotoric Flux Orientation, Serbian Journal of Electrical Engineering Vol. 5, No. 2, Nov. 28, 27 227. [3] M. Marcu, I. Utu, L. Pana, M. Orban: Computer Simulation of Real Time Identification for Induction Motor Drives, International Conference on Theory and Applications of Mathematics and Informatics - ICTAMI 24, Thessaloniki, Greece, pp. 295 35. [4] A. Chikhi, M. Djarallah, K. Chikhi: A Comparative Study of Field-oriented Control and Direct-torque Control of Induction Motors using an Adaptive Flux Observer, Serbian Journal of Electrical Engineering Vol. 7, No., May 2, 4 55. [5] R.B. Gimenez: High Performance Sensorless Vector Control of Induction Motor Drives, PhD Thesis, University of Nottingham, Dec. 995. 83
K. Negadi, A. Mansouri, B. Khatemi [6] C. Schauder: Adaptive Speed Identification for Vector Control of Induction Motor without Rotational Transducers, IEEE Transactions on Industry Applications, Vol. 28, No. 5, Sept/Oct. 992, pp. 54 6. [7] M. Messaoudi, L. Sbita, M.B. Hamed, H. Kraiem: MRAS and Luenberger Observer based Sensorless Indirect Vector Control of Induction Motors, Asian Journal of Information Technology, Vol. 7, No. 5, 28, pp. 232 239. [8] M. Djemai, J. Hernandez, J.P Barbot: Nonlinear Control with Flux Observer for Singularly Perturbed Induction Motor, IEEE Conference on Decision and Control, Vol. 4, San Antonio, TX, USA, Dec. 993, pp. 339 3396. [9] S. Drakunov, S. Utlun: Sliding Mode Observers, 34th IEEE CDC, New Orleans, LA, USA, 995, pp. 3376 3378. [] A.S. Iong, L. Gelman: Advances in Electrical Engineering and Computational Science, Lecture Notes in Electrical Engineering, Vol. 39, Springer, 29. 84